Command-line (bash/GIMP) mass conversion from XCF to JPG

The purpose

I use GIMP a lot. I store the images in the native file format, XCF. Now I’m stuck with a lot of files I can’t see outside GIMP, but I don’t want to save those files as anything else, because I’ll lose all the layer data. Solution: Batch conversion to JPEG as a simple bash script to run from the command line.

The idea is to make a JPEG copy of the image as it’s seen when it’s opened with GIMP. For that reason, I’ve chosen to flatten the image by visible layers only, and to crop it to image size.

The script

It looks like LISP and it looks like bash. In fact, they’re mixed.

#!/bin/bash
{
cat <<EOF
(define (convert-xcf-to-jpeg filename outfile)
  (let* (
	 (image (car (gimp-file-load RUN-NONINTERACTIVE filename filename)))
	 (drawable (car (gimp-image-merge-visible-layers image CLIP-TO-IMAGE)))
	 )
    (file-jpeg-save RUN-NONINTERACTIVE image drawable outfile outfile .9 0 0 0 " " 0 1 0 1)
    (gimp-image-delete image) ; ... or the memory will explode
    )
  )

(gimp-message-set-handler 1) ; Messages to standard output
EOF

for i in *.xcf; do
  echo "(gimp-message \"$i\")"
  echo "(convert-xcf-to-jpeg \"$i\" \"${i%%.xcf}.jpg\")"
done

echo "(gimp-quit 0)"
} | gimp -i -b -

To try it out, simply execute the script from a directory containing several .xcf files. Be sure not have any .jpg files you care about in the same directory, because the outputs are with the same file names, just with the .jpg extension (old files are overwritten with no warning).

You will get kind-of-warning messages while the script is iterating, indicating which file is being processed. This is normal.

The concept of this script is simple. An ad-hoc LISP script is generated in the Bash block, which is enclosed by curly brackets. First we define the function, which converts one file. The bash script then creates calls to this function, by means of the (Bash) for-loop. All this is then fed into GIMP through standard input (piping).

Some LISP notes

I’m not really into LISP. So I ran into some trouble. These are my notes, so I won’t go through it again:

First, there’s the Script-Fu console, which was, well, sort-of helpful. The internal functions’ API can be found there as well.

As a LISP novice, I didn’t know the difference between “let” and “let*”. It turns out, that let* allows the use of previous assignments in the following ones, so this is what you get in the Script-Fu console:

> (let ( (x 2) (y x)) y)
Error: eval: unbound variable: x 

> (let* ( (x 2) (y x)) y)
2

It’s also worth to note, that the GIMP interpreter does not remember functions across different -b command-line arguments:

# First statement succeeds, second fails.
gimp -i -b '(define (myfun x y) (- x y))' -b '(myfun 2 3)'

# This works, because it's two statements in one execution (yuck!)
gimp -i -b '(define (myfun x y) (- x y)) (myfun 2 3)'